Travel outfits, whether by car, by plane or train, are meant to be comfortable. Since leggings are more popular now than they ever were in the 80s, they are my go-to for travel attire. They are so comfortable and easy; I pair mine with a looser top that's longer in the back so it covers my behind. My denim jacket and a scarf are also staples for in-flight comfort since it usually gets a bit chilly in those higher altitudes.
Since I'm heading toward chilly weather, I decided it would be a good idea to wear my heaviest boots on the plane instead of packing them and worrying about the checked bag weight limit. These are great because they easily slip on and off without a lot of effort, which is perfect for getting through the security line without aggravating everyone behind me.
Lastly, I always make sure to carry a large tote for my carry-on. That way, I can stuff any extras that don't go in my suitcase into it as well as my entertainment items.
C. Wonder Tote (no longer available online)
Denim Jacket-AE (similar)
Rain Boot-Chooka (from DSW)